I have a d600 and I appreciate very much the high resolution I get (useful especially for landscapes), but for most other subjects I find 6000 x 4000 to be overkill, making post-processing unnecessarily intensive, computationally.

I was thinking of shooting at 4500 x 3000 and was wondering about quality. How does the camera resample the 24MP to the ~13MP? Does it use all available photo detectors and do a filtering of some sort? I was looking for a knowledgeable answer and I could find nothing around.

A reformulation to make it clear: if your end product has a lesser resolution anyway (say, 750 by 500 images for the web), do you think you would lose quality by shooting at reduced size, if you downscale the image anyway, afterwards?
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haelixJan 4 '13 at 16:30

You could consider shooting a smaller JPEG along with your RAW, work on the JPEG and then synchronize your edits. Also, if you are considering resizing your image I would consider using lossy DNG compression instead, though it will not reduce the pixel count.
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Henrik HelmersJan 4 '13 at 16:45

I would like to point out that @Henrik's comment here is really important. Using in-camera downsampling eliminates the ability to do RAW editing. This is the case for either Canon or Nikon cameras, as the simple fact of the matter is when you downSAMPLE, the final outcome is no longer the original "raw" data...it is sampled data. By shooting both RAW+JPEG, editing the JPEG, and syncing edits, you don't lose that full RAW, nor the editing latitude RAW (and ONLY RAW) can provide.
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jrista♦Jan 4 '13 at 23:40

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@haelix: In Lightroom, you can sync the edits from one photo to one or more others. It is simply called "Sync" in the menu and button options. You edit the JPEG to look how you want...it'll probably not look perfect, you can't edit JPEG as broadly as you can RAW...but once you have exposure, white balance, etc. correct, you can sync those edits to the RAW.
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jrista♦Jan 7 '13 at 18:03

3 Answers
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All photosites are used and the image is down-sampled by software running on the camera. This gives higher image quality than skipping pixels which is what happens in video mode and why you will easily see moire artifacts in videos but not in images.

Some cameras use even more clever processing combined with a special arrangement of pixels to make down-sampling easier given the interleaved layout of the Bayer filter use on most cameras.

D600 allows shooting in DX mode and that doesn't down-sample as far as I know, it's literally a smaller region of the sensor.
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John Cavan♦Jan 4 '13 at 18:55

@JohnCavan - Correct. I assumed the author was not talking about cropping since that 4500x3000 is too high for that. He seems to be concerned about the amount of data. Going into DX mode would reduce it but affect the field-of-view.
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ItaiJan 4 '13 at 19:10

True, but for casual shooting he might not care. At any rate, I suggested it to him as an alternative, he may not have realized that he could do that. I wouldn't bother myself, I'd rather have all the data and decide after the fact.
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John Cavan♦Jan 4 '13 at 19:31

No, this is unrelated to DX mode which is entirely different. I understand your comments, they make sense.
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haelixJan 6 '13 at 9:24

Modern digital cameras offer a wide range of output formats, giving the photographer all sorts of options for file sizes and quality of saved images. There are already some great questions dealing with various aspects of this problem once you start to break it down a bit (links included where I thought they'd be helpful), but I'll take a shot at a high-level roadmap here.

First, your output is going to either be some form of RAW file or a processed file (most likely JPG). Most cameras also give you the option of saving as a TIFF file, but I'll skip that for simplicity (personally, I don't think there's anything there you'd miss).

RAW formats give you the maximum information available from the sensor, delivered straight to you for post-processing using your preferred tools & techniques. In short, if you're doing your own post-processing (beyond stuff like tagging), there's a very good chance you want to be working with RAW files. Some Canon cameras offer multiple size options for RAW files, but I don't believe this is true for the D800 -- if you shoot RAW, you get the full file, no matter what.

If you output to JPG, you're getting an image that's been processed by your camera's onboard processor for all sorts of factors that aren't applied to RAW files, including, but not limited to:

In addition to compression, the image sizes (L/M/S) also apply to JPGs, giving you all sorts of options for reducing the size of the output files. So, which of these settings is best? Unfortunately, it really depends on your needs and the nature of the subject you're shooting. As I mentioned, since the D800 doesn't give you a small-RAW option, the downsampling you'll see in JPG processing is based on algorithms in the Camera's EXSPEED processor.

The algorithms used to create in-camera JPGs are generally very, very good, but the one think you'll want to keep in mind is that processing done by the camera is a one-way trip. If there's anything about those settings that's not quite optimal for any given shot, your post processing is going to be working to repair the product of that processing rather than working directly from the information coming from the sensor. That's why I believe in most cases, if you're planning on post-processing images, you'll want to work from a RAW file.

With the D600, I would suggest shooting in DX mode for those circumstances rather than going through image resampling.

You can find this in the shooting menu of the camera, under image area. There's the Auto DX Crop for when you use DX lenses as well as an option to choose image area (FX or DX), which is defaulted to FX mode for normal shooting. So, if you shoot DX, it literally just uses that region of the sensor and doesn't scale the image down and that would give you 3936 x 2624 which are touch smaller than what you listed but should be cleaner than JPEGs scaled from FX.

@haelix - Not quite. It will have the angle of view of a different focal length, but it's not going to be any more information than shooting FX and cropping to the same region. At any rate, this is your only viable option if you want to shoot raw and reduce the size in camera.
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John Cavan♦Jan 6 '13 at 14:20